Option 1: "usar" is not the correct answer in this context. "Usar" refers to a type of soil found in arid regions.

Option 2: "reh" is the correct answer. It refers to the white salt which covers the land in some areas during the dry season. This phenomenon is commonly seen in desert regions.

Option 3: "erg" is not the correct answer for this question. "Erg" is a term used in geography to refer to a large expanse of sand dunes.

Option 4: "reg" is not the correct answer for this question. "Reg" is a term used in geology to refer to a desert surface covered with gravel and stones.

In conclusion, the correct answer is option 2, "reh," which specifically refers to the white salt covering the land during the dry season.